The present invention relates to an apparutus for generating a tracking servo signal in an apparatus for reading recorded information.
There has been known a tracking servo control unit in an apparatus for optically reading recorded information which apparatus has such a construction, for example, as shown in FIG. 1. That is, the light beam radiated from a laser light source 1 enters a recording surface on a recording disc 6 carrying information through a lens 2, beam splitter 3, 1/4-wave length plate 4, and object lens 5. The laser light is converged by the object lens 5 to be a pickup light spot as a minute information detecting point on the recording surface. The light reflected by or passed through this disc 6 is separated by the beam splitter 3 and irradiated onto the respective light receiving surfaces of a pair of photoelectric conversion elements 9a and 9b. The out-puts of both photoelectric conversion elements 9a and 9b are applied to a differential amplifier 12 through amplifiers 10a and 10b and LPFs (low-pass filters) 11a and 11b, respectively. This differential output is input to a driving amplifier 14 through an equalizer 13 to be a driving signal for a driving coil 8 for moving the object lens 5 in the direction perpendicular to the track.
The pair of photoelectric conversion elements 9a and 9b are attached so that the receiving surface forms such a dividing line 9c as shown in FIG. 2. The dividing line 9c is in parallel to the tangential direction of the recording track (indicated by an arrow Y) and is disposed so that these elements 9a and 9b are symmetrical with respect to a light axis 7' of the reflected light of the light spot. A reference numeral 7 is a light axis of the incident light, and a reference numeral 15 indicates a spindle motor for rotating the disc 6.
When, with such a construction mentioned above, the center of the pickup light spot deviates in the direction perpendicular to the track from the center of the recording track, the strength distribution of the light entering the photo detectors 9a and 9b becomes asymmetrical depending upon this deviation, resulting in the difference between the outputs of both detectors. Therefore, by obtaining the difference of low-frequency components between the output signals from both these detectors by the differential amplifier 12, a tracking error signal is obtained. By moving the object lens 5 in dependence on this error signal in the direction perpendicular to the track (in the direction of a radius of the disc), the light spot for picking up is deflected thereby to perform the precise tracking operation.
However, such prior art apparatus as mentioned above has a defect in which due to the inclination of the recording surface of the disc 6, deviation of the object lens 5 in the radius direction of the disc due to disturbances, or the like, the center of the reflected light 16 from the disc on the light receiving surfaces of the detectors 9a and 9b deviates from the dividing line 9c as indicated by a broken line in FIG. 2, even when the pickup light spot is aligned on a target track, which causes a critical constant deviation of the light spot from a target track in the tracking servo control system.